Mapgen: Fix biome Y calculation regression

BiomeGen::getNextTransitionY(y) did not guarantee the condition (y < biome_y_min)
of the next loop because the function may return the value (biome_y_min - 1).
Hence, the biome was not updated until one Y coordinate after.

src/mapgen/mg_biome.h

@@ -197,7 +197,8 @@ private:
 	Noise *noise_heat_blend;
 	Noise *noise_humidity_blend;
 
-	// ordered descending
+	/// Y values at which biomes may transition.
+	/// This array may only be used for downwards scanning!
 	std::vector<s16> m_transitions_y;
 };